why?

what a day...why is it whenever i feel really good about something, like my playing, that i crash and burn? i mean, it's so damn frustrating...like today, we played the Beethoven in forum. I knew I didn't play as well as I have (reeds/nerves...grrr), but Kathy was a little harsh, even though she's right and i probably deserved it. i asked her how it sounded and she said, "it sounded like nothing; i couldn't hear you and when i could, the pitch, rhythm, and tone quality weren't very good. you've got a lot of work to do." ouch. so, needless to say, i just feel sorta shitty, even though she's right. and, damn those stupid nerves. i almost froze up because everyone was watching...why is it i can play a duet almost perfectly with someone and feel great and then bomb an ensemble piece? am i not supposed to feel good about what i do? or am i supposed to feel good, but get negative response? why?